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Implicit Conversion not Found Because of Type Widening #10204

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scabug opened this issue Feb 23, 2017 · 4 comments
Open

Implicit Conversion not Found Because of Type Widening #10204

scabug opened this issue Feb 23, 2017 · 4 comments
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fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/)
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@scabug
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scabug commented Feb 23, 2017

Type widening and implicit resolution does not work well together. The effect is visible if the result of Option.getOrElse must be converted by an implicit conversion. There is a corresponding question on StackOverflow answered by Rex Kerr (http://stackoverflow.com/questions/13186063/option-getorelse-type-mismatch-error), but I did not find a matching issue.

object Test {

  trait Formatable[-T] {
    def format(t: T): String
  }

  object Formatable {
    implicit val stringFormatable: Formatable[String] = s => s
  }

  case class Formatted(string: String)

  object Formatted {
    implicit def formatted[T](t: T)(implicit ev: Formatable[T]): Formatted = Formatted(ev.format(t))
  }

  val a = Option("b")

  // type mismatch;
  //found   : java.io.Serializable
  //required: com.bosch.inst.base.etc.StringInterpolationTest.Formatted
  val formatted: Formatted = a.getOrElse("b")


}
@scabug
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scabug commented Feb 23, 2017

Imported From: https://issues.scala-lang.org/browse/SI-10204?orig=1
Reporter: Stefan Wachter (swachter)

@scabug
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scabug commented Mar 15, 2017

@SethTisue said:
I have a feeling this is "not a bug", but someone would have to delve into the spec to be sure. I'm glad to have the ticket to document the issue, but it isn't actionable without an analysis.

@scabug scabug added the quickfix label Apr 7, 2017
@scabug scabug added this to the Backlog milestone Apr 7, 2017
@soronpo
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soronpo commented Jun 16, 2017

Compiles in dotty:
https://scastie.scala-lang.org/soronpo/fOnryKGiRJych5zPMLKp7w

@KuceraMartin
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Any updates about this? Here is an even more minimized example:

//> using scala 2.13

object MyApp {

  trait Conv
  implicit def convInt(s: Int): Conv = ???
  def f[B >: Int](v: B): B = ???
  def g(c: Conv): Unit = ???
  g(f(42))

}

Output:

[error] type mismatch;
[error]  found   : Any
[error]  required: MyApp.Conv
[error]   g(f(42))
[error]     ^^^^^
Error compiling project (Scala 2.13.11, JVM)
Compilation failed

(compiles in Scala 3)

@SethTisue SethTisue added the fixed in Scala 3 This issue does not exist in the Scala 3 compiler (https://github.com/lampepfl/dotty/) label Nov 28, 2023
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